My Partner's Kink Caught Me Off Guard

I’ve been in this open relationship for about two years now. It’s great because I get to explore my kinks with different people while still maintaining a loving, intimate connection with my partner Alex. We trust each other and are both very comfortable talking openly about our desires and boundaries. Recently, though, something came up that left me feeling a bit uncertain. One of the girls I’ve been seeing has a specific interest in...let's say 'age play'. This wasn’t entirely unexpected, but when she brought it up with me explicitly, it sparked something within her that I had never seen before, this playful, youthful innocence that she let out. It was intriguing to see her transform like that and it made me realize just how much pleasure and fun this kink could bring for both of us. But then Alex asked me about my nights with her and the conversation took an unexpected turn. I hesitated when I told him about our session, knowing that some of these dynamics might be a bit out of his comfort zone. We’ve had similar discussions before where we agreed to not push each other into anything too uncomfortable. The thing is, this girl has piqued something in me as well, something new and exciting, and I want to explore it further with her. But how do I explain that excitement without making Alex feel left out or questioning our relationship? It’s a delicate balance but one I need to navigate if I’m going to continue enjoying my sexual freedom while keeping the trust between us intact.